Because of advanced computerization, and development of information communication and multimedia technologies in recent years, demands for a higher density and a greater capacity of optical information recording medium is increasing. The upper limit of the recording density of optical information recording medium is limited mostly by a spot diameter of an optical beam which records or reproduces information. A light spot diameter is substantially expressed by λ/NA, where λ expresses a wavelength of light source, and NA expresses a numerical aperture of an objective lens. To improve the recording density, the light spot diameter needs to be reduced.
However, because of absorption by optical elements and the limit of sensitivity property of a detecting device, the wavelength λ of the light source cannot be reduced less than a wavelength of an ultraviolet range. Moreover, the improvement of NA is substantially limited by the permissible range of the inclination of a medium. On this account, there is a limit in improving the recording density by reducing the light spot diameter.
To overcome the limit, there is a technique called a super-resolution medium technology, which achieves reduction in effective light spot diameter by using the optical characteristics of recording medium. According to the super-resolution medium technology, a part of the light spot is masked by utilizing (a) a temperature distribution on the recording medium and (b) a change in transmittance, which are induced by the light spot, so as to reduce the effective spot for recording/reproducing, thereby increasing a recording/reproducing density. Such an effect is called a medium super-resolution effect.
FIG. 20 schematically illustrates the above-described medium super-resolution effect. A light spot 111 relatively scans the super-resolution medium in a direction indicated by an arrow 113 to carry out recording and reproducing. In general reproduction, all recorded marks 112 positioned in the light spot 111 contribute to reproducing signals. However, in the super-resolution medium, the light spot 111 is masked except for the central region 111a with a high light intensity, so that only the recorded mark 112a in the central region 111a is reproduced. In this way, the diameter of effective light spot for reproducing is practically reduced. On the contrary to the example of FIG. 20, it is also possible to detect the recorded marks 112 in a peripheral region 111b in the light spot 111 by masking the central region 111a. 
The followings are conventional methods for carrying out the super-resolution medium technology.
(1) A super-resolution read-out technology using a mask made of an organic dye
(2) A super-resolution technology using a photochromic mask film
(3) A super-resolution technology using an inorganic oxide film
Among these, the method using an organic dye or a photochromic (the methods (1) and (2)) is not sufficiently reliable in information reproduction since a disk including an organic mask layer is easily broken by heat, and is endured readout of approximately 10,000 times or less. For this reason, those methods are not in practical use. In addition, because of the fragility under heat, those methods cannot be employed for rewritable discs.
In contrast, referring to Document 2, a disk according to the super-resolution technology using the inorganic oxide film (the method (3)) allows (repeated) readout of 100,000 times or more, and a phase-change medium applying this super-resolution film is rewritable. This is because, a disk using super-resolution film of an inorganic material is not broken by heat as easily as the disc using the super-resolution film of an organic material, such as the mask using an organic dye or the photochromic mask layer. On this account, the inorganic oxide super-resolution film according to the method (3) is a super-resolution material applicable to both a read-only disc and a rewritable disc.
Further, Document 1 discloses the optical information recording medium which is arranged such that (i) a Co—Si—Na—Ca—O film or a Co3O4 film is used as the inorganic oxide super-resolution film, and (ii) reflectivity of a multilayer film increases with an increase in intensity of the incident light. This optical information recording medium is made in view of such a defect of an inorganic oxide super-resolution film that a decrease in reflectivity with a change in complex refractive index causes an increase in diameter of an effective reproducing spot, thereby failing to ensure a reproducing signal amplitude property, which contributes to improvement in recording density.
Moreover, according to Document 1, when the inorganic oxide super-resolution film (hereinafter referred to as an inorganic super-resolution film) is irradiated with a laser light greater than a certain threshold value, a complex refractive index thereof changes. When applying the inorganic super-resolution film to an optical disc, the inorganic super-resolution film is provided as a part of a multilayer lamination structure. In this structure, when reproducing information recorded on the optical disc, the complex refractive index of the inorganic super-resolution film changes in the center of the light spot where the temperature has increased, and the complex refractive index of this area changes due to a multiple interference of light in the multilayer film. This enables intensive readout of signals in a part of the light spot, thereby reducing the effective spot diameter for reproducing.
Such a multilayer structure containing an inorganic super-resolution film can be improved in function by increasing the change in complex refractive index of the multilayer structure. In order to increase the change in the reflectivity of the multilayer film, it is effective to adequately use the multiple interference of light in the multilayer film.
However, one of the inorganic super-resolution films used in Document 1 is 50 nm in thickness, with a complex refractive index (=n−ki, where i is an imaginary unit) n=2.48, and an extinction coefficient k=0.48, as default values. These values are respectively changed to: n=2.41 and k=0.57 with an increase in intensity of incident light. However, since the extinction coefficient k is large in this structure, there is a limit in effectively increasing the change in the reflectivity of the multilayer film.
That is, in the arrangement in which the extinction coefficient k is such a large value, light is absorbed as it passes through the inorganic super-resolution film, so that the inorganic super-resolution film functions as a translucent film. When the inorganic super-resolution film functions as a translucent film, the light is absorbed while multiple optical interference repeatedly occurs. Thus, the effect of multiple optical interference is not fully ensured.
To more specifically explain this principle, the following explains light absorption in the above-described inorganic super-resolution film with a thickness=50 nm, and an extinction coefficient k=0.48. For ease of explanation that only deals with absorption, the multiple interference is ignored here. Thus, the intensity of transmitted light which passes through the inorganic super-resolution film is expressed by the following equations:I=I0×exp(−αx),
where I0 expresses the intensity of incident light, I expresses the intensity of transmitted light, x expresses the thickness of the film, and a expresses the absorption coefficient, which is denoted by α=4πk/λ, where λ is the wavelength of the incident light.
According to these equations, the intensity I of the transmitted light exponentially decreases as the thickness x and the extinction coefficient k increase.
In the above example, the wavelength A of the light source is 660 nm, which makes the transmittance (=the intensity of transmitted light/the intensity of incident light) 63% according to the foregoing equation. However, because the density of the optical information recording medium is increasing in recent years, the wavelength of the light source tends to decrease, and an optical information recording medium using a blue laser of wavelength 400 nm is now put to practical use. When the light source with a wavelength=400 nm is used in the above example, the transmittance decreases to 47%.
Note that, in this calculation, the multiple interference is ignored to only deal with the absorption. However, if considering the multiple interference, the transmittance further decrease because respective rays of the light weaken each other.
Therefore, when the above structure uses the optical system which emits light of wavelength 400 nm, the amount of light having passed through the inorganic super-resolution film becomes equal to or less than one half of the incident light. This amount is not enough to utilize the multiple interference. In addition, the light is not efficiently used.
The change in the reflectivity of the multilayer film can also be increased by increasing the thickness of the inorganic super-resolution film so as to enhance the effect of change in refractive index. However, when a translucent inorganic super-resolution film is increased in thickness, light cannot pass through the inorganic super-resolution film. The increase in thickness of the inorganic super-resolution film is therefore not desired.
According to the above equations, the transmittance drastically decreases from 63% to 40% for a light source with a wavelength=660 nm, from 47% to 22% for a light source with a wavelength=400 nm, when the thickness is doubled, that is, when the thickness is increased to 100 nm. As the transmittance thus greatly decreases for a short wavelength, it is insufficient to ensure the effect of multiple interference. In addition, the light is not efficiently used.
Especially, when the inorganic super-resolution layer is translucent, the efficiency in use of light decreases in the information layer including the inorganic super-resolution film. Therefore, there is a difficulty in using such an arrangement for a multi-recording-section structure, that contains lamination of a plurality of recording layers for recording information, or of a plurality of recording surfaces where information are recorded in the form of pits which create depression/projection on the recording surface.
Note that, in order to increase the change in reflectivity without changing the thickness of inorganic super-resolution film, one possible method is increasing the amount of change in the complex refractive index of inorganic super-resolution film; however, since the amount of change in the complex refractive index differs for each substance, it is difficult to expect a significant improvement by this method.
(Documents)
1. Japanese Laid-Open Patent Publication No. 2001/84643 (Tokukai 2001-84643, filed on Mar. 30, 2001), corresponding U.S. Pat. No. 6,524,766
2. Toshimichi Shintani, Motoyasu Terao, Hiroki Yamamoto and Takashi Naito, “Jpn. J. Appl. Phys (Japan Journal of Applied Physics) Vol. 38”, 1999, p. 1656 to p. 1660
3. Kiyoshi Takahashi, Yoshihiro Hamakawa and Akio Ushirokawa, “Photovoltaics”, second impression of the first edition, MORIKITA SHUPPAN, published on Dec. 10, 1980, p. 41